The presence of heavy metals in wastewater and surface water is becoming a severe environmental and public health problem. The removal of heavy metals from both natural water supplies and industrial wastewater streams is becoming increasingly significant as awareness of the environmental impact of such pollutants is fully realized. In particular, copper, nickel and chromium found in wastewaters are designated as priority pollutants by the United States Environmental Protection Agency (EPA). Among them, hexavalent chromium is of special concern due to its high toxicity. The vast majority of toxic metals are waste products of industrial and metallurgical processes. The effluents from electroplating plants, metal finishing operations and extractive metallurgy processes contain especially high concentrations of dissolved metals. Metal ion containing waste solutions are commonly found at levels that exceed acceptable disposal limits. These pollutant concentrations must be reduced to meet the legislative standards, and recovered where feasible.
Various techniques are available for the removal of heavy metals from wastewater. Chemical precipitation is the most common method of metal removal for wastewater with high strength of heavy metal. The chemicals most frequently used for precipitation of metals are lime, caustic, and sodium carbonate. Although most heavy metals are precipitated readily by pH adjustment, hexavalent chromium is highly soluble and does not precipitate out of solution at any pH. Consequently, treatment for chromium usually consists of a two-stage process: first, the reduction of hexavalent chromium to the trivalent form; and, second, the precipitation of the trivalent chromium. Cr(VI)-bearing streams are segregated and treated separately; the reduced chromium-containing effluent can then be blended with other metal-contaminated streams for further treatment. An additional problem relating to handling the copious quantities of sludge produced is the disposal of sludge, which is again an economical and environmental issue. Therefore, recovery of the chromium present in industrial wastewater is necessary for economical and environmental reasons.
Attention has thus turned to non-consumptive methods which include ion exchange and adsorption process. The former process for selective removal and recovery of metals has been used for years. However, the high capital and operating costs make this process economically unfeasible, unless the treated effluent can be recycled and reused. Adsorption has emerged as a cost-effective technique for removing metals from wastewater, and has been widely studied during recent decades. In principle, adsorption not only can remove heavy metals but also can recover and recycle them back into the industrial process. The most commonly used adsorbent for heavy metal removal is activated carbon. But the potential problems are the high regeneration cost and the loss of adsorption capacity due to attrition during the regeneration process in which the used carbon needs to be regenerated at high temperature (near 1000° C.) commonly used by commercial regenerators. In addition, part of the carbon may be destroyed by chemical treatment. Taken together, regeneration of the solids is relatively uneconomical compared to disposal, and the resultant carbon wastes become a secondary pollution stream.
The application of magnetic particle technology to solve environmental problems is one of several new and innovative methods that have received considerable attention in recent years. Magnetic particles can be used to adsorb contaminants from aqueous effluents, and after the adsorption the adsorbent can be separated from the solution by a simple magnetic process. Furthermore, the adsorbed metals can be concentrated into a small volume by stripping the metals off the surface using an appropriate stripping agent for reuse, and permits recovery of the magnetic particles. However, most of these adsorbents have the disadvantages of small adsorption capacity or slow adsorption rate, due to small surface area or porous properties (respectively), which limits their use in wastewater treatment. In addition, recovery of these adsorbents for reuse is unsatisfactory.
Thus, it would be a significant advancement in the art to provide an improved method and superior material for treating industrial wastewater. It is an object of the invention to provide such a system that would be able to remove heavy metals without producing any sludge. It would be desirable to provide such method which can regenerate the adsorbent for reuse and also recover the valuable metals for industrial purpose. It is further object to provide method which is less costly and simpler to operate and less costly than conventional apparatus.